This invention generally relates to improved fuel cell systems, and more particularly to fuel cell assemblies containing substantially planar arrays of fuel cells or membrane electrode assemblies (MEA's) with a relatively uniform anode-side hydrogen gas supply.
Fuel cells are devices that directly convert chemical energy of reactants, i.e., fuel and oxidant, into direct current (DC) electricity. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuel, as well as portable power storage, such as lithium-ion batteries.
In general, fuel cell technology includes a variety of different fuel cells, such as alkali fuel cells, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, and enzyme fuel cells. Today's more important fuel cells can be divided into several general categories, namely (i) fuel cells utilizing compressed hydrogen (H2) as fuel; (ii) proton exchange membrane (PEM) fuel cells that use alcohols, e.g., methanol (CH3OH), metal hydrides, e.g., sodium borohydride (NaBH4), hydrocarbons, or other fuels reformed into hydrogen fuel; (iii) PEM fuel cells that can consume non-hydrogen fuel directly or direct oxidation fuel cells; and (iv) solid oxide fuel cells (SOFC) that directly convert hydrocarbon fuels to electricity at high temperatures.
Compressed hydrogen is generally kept under high pressure and is therefore difficult to handle. Furthermore, large storage tanks are typically required and cannot be made sufficiently small for consumer electronic devices. Conventional reformat fuel cells require reformers and other vaporization and auxiliary systems to convert fuels to hydrogen to react with oxidant in the fuel cell. Recent advances make reformer or reformat fuel cells promising for consumer electronic devices. The most common direct oxidation fuel cells are direct methanol fuel cells, or DMFC. Other direct oxidation fuel cells include direct tetramethyl orthocarbonate fuel cells. DMFC, where methanol is reacted directly with oxidant in the fuel cell, is the simplest and potentially smallest fuel cell, and also has promising power application for consumer electronic devices. SOFC converts hydrocarbon fuels, such as butane, at high heat to produce electricity. SOFC requires relatively high temperatures in the range of 1000° C. for the fuel cell reaction to occur.
The chemical reactions that produce electricity are different for each type of fuel cell. For DMFC, the chemical-electrical reaction at each electrode and the overall reaction for a direct methanol fuel cell are described as follows:
Half-reaction at the anode:CH3OH+H20→CO2+6H++6e−
Half-reaction at the cathode:1.502+6H+6e−→3H20
The overall fuel cell reaction:CH3OH+1.5O2→CO2+2H2O
The overall fuel cell reaction:CH3OH+1.5O2→CO2+2H2O
Due to the migration of the hydrogen ions (H+) through the PEM from the anode to the cathode, and due to the inability of the free electrons (e−) to pass through the PEM, the electrons flow through an external circuit, thereby producing an electrical current through the external circuit. The external circuit may be used to power many useful consumer electronic devices, such as mobile or cell phones, portable music players, calculators, personal digital assistants, laptop computers, and power tools, among others.
DMFC is discussed in U.S. Pat. Nos. 4,390,603 and 4,828,941, which are incorporated by reference herein in their entireties. Generally, the PEM is made from a polymer, such as Nafion® available from DuPont, which is a perfluorinated sulfonic acid polymer having a thickness in the range of about 0.05 mm to about 0.50 mm, or other suitable membranes. The anode is typically made from a Teflonized carbon paper support with a thin layer of catalyst, such as platinum-ruthenium, deposited thereon. The cathode is typically a gas diffusion electrode in which platinum particles are bonded to one side of the membrane.
In a chemical metal hydride fuel cell, sodium borohydride is reformed and reacts as follows:NaBH4+2H2O→(heat or catalyst)→4(H2)+(NaBO2)
Half-reaction at the anode:H2→2H++2e−
Half-reaction at the cathode:2(2H++2e−)+O2→2H2O
Suitable catalysts for this reaction include platinum and ruthenium, among other metals. The hydrogen fuel produced from reforming sodium borohydride is reacted in the fuel cell with an oxidant, such as O2, to create electricity (or a flow of electrons) and water byproduct. Sodium borate (NaBO2) byproduct is also produced by the reforming process. A sodium borohydride fuel cell is discussed in U.S. Pat. No. 4,261,956, which is incorporated by reference herein in its entirety.
Fuel cell systems traditionally have multiple fuel cells arrayed in stacks. These stacks tend to be relatively inefficient, as it is difficult for the fuel to flow uniformly to the anode sides of fuel cells.
The patent literature discloses attempts to improve fuel flow in the stack. In one example, U.S. Pat. No. 6,887,611 B2 discloses a flexible external fuel cell gas manifold designed to accommodate bowing and shrinkage of the stack while maintaining a gas seal, and reducing dielectric insulator breakage.
U.S. Patent Application Publication No. 2005/0196666 A1 discloses fuel cells that are assembled into substantially planar arrangements. These substantially planar fuel cell arrays can be contour-molded to a desired shape or can be constructed as a pliable fuel cell or as an array of flexibly connected individual fuel cells that overall has a curvilinear shape. However, this publication does not disclose a way to distribute fuel to the fuel cells, but discusses a fuel gel that vaporizes through a fuel permeable layer.
U.S. Patent Publication No. 2005/0255349 A1 also discloses a substantially planar fuel cell array. This reference discloses a hydrogen manifold having two narrow hydrogen channels and two hydrogen ports.
Hence, there remains a need for improved gas manifolds to deliver fuel to the fuel cell.